Put external headers before internal ones. While there, replace a lot of
occurences of "openlibm.h" with <openlibm.h>. It should be thought of as
an external header, as it is installed along with the library.
OpenLibm has an implementation of fenv.h internally. This may be
problematic in case you want it to build against the host system's
implementation, as it would require you to somehow take the fenv.h file
out of the compiler search path.
Simply use a different naming scheme, similar to openlibm.h and
openlibm_complex.h. If we want to build against the host's fenv.h, we
can simply add an '#include <fenv.h>' from within this header.
This test is also present in FreeBSD's <machine/ieeefp.h>. For FreeBSD
it makes sense, but for a portable math library, we cannot assume that
the system has a header file like <sys/cdefs.h> and that it uses a
common header guard.
skip END and add .drectve export in assembly versions
uncomment __strong_reference definition from cdefs-compat.h
use weak references in place of strong references on Mac
add DLLEXPORT to all strong references
Fixes Julia issue #6777
